Courtesy Viv Robins(NEW YORK) -- Viv Robins became part of a clinical trial for tamoxifen, an estrogen blocker used to treat and prevent breast cancer, because her mother had died of the disease at age 54. But the only cancer Robins said she knows she can now avoid for sure is ovarian cancer.

Two years into the tamoxifen trial, when Robins was 46, her ovarian cyst burst, she developed peritonitis and doctors had to remove her uterus and both ovaries to save her life.

"I felt immediately extremely ill," Robins, now 65, who lives in London, told ABC News, recalling the flashing lights in the ambulance as she was rushed into surgery for an emergency 3 a.m. hysterectomy at the Royal Marsden hospital. "The surgeon opened me up and found a stomach full of blood. He didn't know what was going on."

Now, nearly two decades later, as the United Kingdom's National Institute of Health and Clinical Excellence is drafting guidelines that would recommend tamoxifen as a preventive drug for women at high risk for breast cancer -- the U.S. Food and Drug Administration approved tamoxifen for women at high risk in 1998 -- Robins said she can't help but wonder whether it was the drug that prompted her cyst to burst. She'd had benign ovarian cysts since she was 19, she said, and they had never given her problems until she started taking tamoxifen.

Dr. Len Lichtenfeld, the deputy chief medical officer at the American Cancer Society, said, "Tamoxifen has been used for such a long time by so many women around the world that, were this a serious complication, it should have shown up. It's not one that rises to common knowledge."

In the years that followed the clinical trial, Robins said she continued to keep a watchful eye on documents mailed to all trial participants. Several studies about cyst development in some post-menopausal patients caught her eye, but no findings linked tamoxifen to the rupturing of ovarian cysts.

More common adverse effects have included blood clots and endometrial cancer, but Dr. Kimberly Blackwell, a breast cancer specialist at Duke University Medical Center, said these risks were on par with those associated with taking birth control pills.

The U.S. Preventive Services Task Force recommends that clinicians discuss tamoxifen with their high-risk patients who are at low risk of developing side effects, though it is not a sweeping recommendation like the one that may come to pass in the U.K. The task force also recommends not prescribing tamoxifen to patients with a low or average risk of developing breast cancer.

"I wish the drug was more widely emphasized," said Blackwell, "because it's a known way of preventing breast cancer for women at high risk of developing it."

Tamoxifen reduces the likelihood that high-risk women will develop breast cancer by about half, she said, referring to findings from the Breast Cancer Prevention Trial, which led to tamoxifen's FDA approval.

Factors that increase the risk of blood clots include smoking and being overweight, so those patients would not be ideal candidates for tamoxifen.

"If the risk [of developing breast cancer] is high enough, doubling the risk of blood clots, many times, is worth trying to take the medication," Blackwell said. "It should be a discussion between every woman and her physician, and every woman should know her own individual risk factors for breast cancer."